The creation of a solid chassis that fulfils the requirements for stability and mass in steep and challenging terrains, which can consequently be upgraded with expansion stages in different ways.
For the fulfilment of the aforementioned criteria, the chosen type of chassis was the so called rocker boogie chassis – the same kind currently used by rovers on Mars, as they have to deal with similarly challenging ground. This type of chassis comprises two chassis “arms” of three wheels each, connected with a differential rod, which grants decent ground contact and traction in nearly all circumstances.
Note: all parts of the chassis have been devised, constructed and manufactured in house
After the angles in the construction had been optimised and the final length of the profiles had been determined by building and testing a prototype, 4 parts were manufactured in customised moulds. In order to minimise weight while maximising the robustness of the construction, the only material used was carbon fibre. The hinged mounting of the individual parts required special slide bearings manufactured with a lathe. Brass and steel were used for said bearings, because this combination of materials guarantees good running properties even when lubricant is lacking. The differential rod is made from carbon fibre as well and is tethered to the chassis with threaded rods and ball joints. This differential prevents tilting of the base plate, which is mounted in only two points, and provides decent ground contact of the wheels while keeping the base plate somewhat level even in difficult terrain.
Steering, motor mounts, wheel mounts
The wheel mounts and the connected motors constitute the second big part of the chassis. The driving wheels are connected to the motors via a hexagonal titanium mount, which in turn is mounted on 40 x 40 mm aluminium profiles. Those profiles are bolted to a kind of lying U, which in turn is vertically pivoted on the chassis, which constitutes the steering. The steering itself is accomplished with the use of servos that are mounted on the chassis and position the driving part via threaded rods. The lying U consists of carbon fibre around a core of Kevlar in a honeycomb structure and is about 10 mm thick. On the inside of the U there is an additional 10 mm semicircular carbon fibre bar for further strengthening.
After completing the design process, the manufacturing of the bodywork could begin. The chosen process worked as follows:
A layer of fibre reinforced material is applied to a positive mould and removed from the mould after curing. The thereby obtained bodywork is thin and hollow, not dissimilar to the one of conventional toy car.
At first, the mould was constructed from styrofoam components that were milled into the desired shape and glued together. The side walls were bevelled to an angle of ten degrees using a hot wire as cutting tool. The basic mould has to be sanded, puttied and sanded again, before a thin layer of fibre glass can be applied, which finishes the mould. After that the mould is coated in a release agent so that the actual bodywork assembly may begin, which comprises adding a thicker layer of fibre glass and a more decorative final layer of synthetic twill. Finally, after laminating the bodywork and separating it from the mould, it is reinforced with ribs in order to provide a solid footing for sensors and cameras.
The chassis as well as the differential are anchored in the base plate. The base plate is made out of a 6 mm plywood board framed by 20 x 20 mm aluminium profiles, in which the chassis is anchored. The thereby obtained “tub” forms the basis, on which the remaining rover is built up.
In order to protect sensible electronics from dirt and mechanical damage, they are integrated in an own board, which is mounted directly in, respectively on the tub. The bodywork leaves about 15 cm of height on the inside, which means that there is room for a second level. This additional board will contain the whole sensor array. Both boards are designed with CAD software and machined in a CNC mill, to ensure accuracy of fit.